A calm, pleasant night, the sky full of stars. Suddenly, without warning, one of them races across the sky, tracking a course over dozens of miles of atmospheric real estate in the blink of an eye. Another follows, then another: the moving bodies aren’t stars at all, but meteors, a whole shower of them. Such showers are among the greatest rewards the night sky holds for those patient enough to watch and wait.

The asteroid Lutetia as seen from the Rosetta satellite. Credit: ESA 2010 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

Then, of course, there are the more dramatic appearances of lights in the sky, such as the one the good people of Siberia witnessed just last month—the one that led to more than 1,200 injuries and led the more alarmed of those people, not without reason, to conclude that the world was coming to an end.

The smaller flashes of light that signify a meteor shower are common, marking the arrival into the Earth’s atmosphere of pieces of rock and metal that have come hurtling across the heavens over millions of miles and years. Most of these meteors, as they are called, burn away in the upper atmosphere; occasionally they land on the planet’s surface, whereupon they are called meteorites. In whatever case, these visitors are bits of real estate that have broken off from still larger stones, called asteroids, so called because they appeared to be like stars to their early observers.

Some asteroids are very large indeed; about sixty of them are more than 100 miles in diameter, while the largest of the more than 17,000 named asteroids, Ceres, measures nearly 600 miles across. Untold millions more, most found in a great belt between Mars and Jupiter, are smaller, ranging from boulder-sized stones to veritable mountains in space.

Only in the last century, however, with the advent of powerful telescopes and spacecraft, have scientists been able to understand much more about the origins and behavior of asteroids. What they’ve learned has told us much about our own planet—and turned up many surprises.

One of them concerns the origin of the Moon. Scientists have long wondered about this little planet, the Earth’s only natural satellite, and in the eighteenth century many believed that the Moon had somehow once been a part of our own planet, perhaps torn away by some giant flood or cataclysm and cast out into space. Later scientists dismissed this, arguing instead that the Earth had somehow lured the Moon into its orbit and kept it there—a sensible enough argument in a time when countries were busily conquering other countries. Today, the prevailing scientific view is that early in our planet’s history, an asteroid the size of Mars—thus more accurately referred to as a protoplanet—collided with the Earth and sent a vast cloud of sandy fragments into space; about 4.5 billion years ago. These fragments then coalesced into the Moon.

Happily, this collision left the Earth’s metallic core more or less intact, which accounts for the abundant presence of metals on our planet. So, too, do meteorites, which have introduced near-surface metals around the Earth, one reason geologists have been so keen to locate meteorite craters in the last few years—and one reason entrepreneurs have been dreaming up ways in which to send spacecraft to mine asteroids as they whirl about in space.

Still another surprise comes from the role of asteroids in making the world safe for humans—if, that is, we follow a very indirect route of reasoning. Sixty-five million years ago, at the end of the Cretaceous period, an asteroid measuring more than five miles in diameter struck the Earth, landing in what is now the Yucatán Peninsula of southern Mexico. When it struck, the asteroid created a crater more than 25 miles deep and 100 miles wide, sending a ball of vaporized rock high into the atmosphere. As it returned to Earth, this flaming debris touched off huge fires that in turn shrouded the planet in ash, plunging it into cold and darkness and, in the bargain, driving some 80 percent of the planet’s living species into extinction. These included most of the dinosaurs—but not a line of small, ground-dwelling mammals that, evolutionary biologists believe, were the distant ancestors of our own species, which may never have had a chance had the great reptiles held sway.

Are we in danger of further collisions, and of worse luck? Perhaps so. In January 1991, an Apollo asteroid—that is, one of a class of asteroids whose orbit crosses the Earth’s regularly—came within 150,000 miles of our planet, close enough to give some asteroid-watchers cause for concern. Nearly 5,000 of these Apollo asteroids have been identified, and some scientists are pressing for a more comprehensive study to calculate their orbits against the possibility of one day having to turn them away from the Earth, a scenario exploited in the 1999 film Armageddon. Even one asteroid a half-mile or so across could produce an explosion as large as that of thousands of hydrogen bombs, triggering a disastrous wave of events and extinctions. Given that possibility, it’s small wonder that NASA scientists were so closely engaged in following asteroid 2012 DA14 last month, since it came within 17,250 miles of us—uncomfortably close in the cosmic scheme of things.

We have more to fear just walking across the street, but even so, asteroid watchers keep their eyes peeled on the heavens just in case.